This invention relates generally to imaging lens systems used in electrophotographic copy devices reproducing images on plain paper and more particularly relates to imaging lens systems used in inexpensive, small, lightweight electrophotographic devices providing high quality copies.
Electrophotographic imaging apparatus such as copying machines are well known in the art. Generally, these machines include an electrophotographic member that is transported past or through a closed-loop series of work stations. The work stations comprise a charging station at which a uniform charge is placed on the photoconductive surface of the electrophotographic member, an exposure station at which the charged surface is exposed to a light image of the document to be reproduced or copied to form thereon a latent electrostatic image of the document, a developing station at which the latent image is developed or made visible by the application of finely divided toner particles that adhere to the member surface according to the latent image configuration and a transfer station at which the toned image is transferred to a record medium or secondary carrier material such as paper.
It is desired to make such a copying machine inexpensive, small, and lightweight, yet producing high quality copies. These requirements have been though to be difficult to attain. A copying machine that achieves these advantages is disclosed, however in U.S. Pat. No. 4,236,807.
It has been determined advantageous in that patent to employ an electrophotographic member in the form of an endless belt mounted on a plurality of rollers and to expose the member to the light image of the document to be copied at a location where the belt is in a flat condition. The light image of the document is projected onto the belt through an optical system that advantageously uses a folded optical path. Such a folded path optical system requires reduced space interior of the copier cabinet and contributes to the desired requirement of a small copier.
The folded path optical system comprises a horizontally disposed, transparent copy or object platen, an illumination system of lamps and reflectors for illuminating the platen, a first right-angle folding mirror disposed below the platen, a lens system aligned laterally of the first folding mirror and a second right-angle folding mirror aligned laterally of the lens system below the exposure station for the electrophotographic member. The object platen and exposure station thus are arranged laterally of one another in substantially parallel planes with the folded optical path therebelow. This is disclosed in U.S. Pat. No. 4,264,199 which further discloses a unitary mounting structure to mount in alignment the various optical system components.
The optical system provides, gathers and transmits light energy reflected from the original document to the electrophotographic member to form the latent electrostatic image thereon. The light image of the document must be of a sufficient quantity fully to discharge areas of the charged member and must be of a high quality to provide a clear, crisp reproduction. The illuminating means thus must provide sufficient light energy evenly distributed across the surface of the copy platen and the lens system must gather and transmit a sufficient quantity of the light image reflected from the document with a sufficient clarity to form the desired latent image on the member.
These requirements have been difficult to satisfy with conventional lens systems, an example of which is disclosed in U.S. Pat. No. 3,507,560 to Baker.
U.S. Pat. No. 3,507,560 discloses a 1:1 magnification, moderately wide 60.degree. field angle lens having an aperature with an f-number of f/11.0. It uses glass stock of medium refractive index and uses four lens elements having mild surface curvatures to attain an acceptably low cost lens system.
Conventional lenses of this range of field angles are well corrected for aberrations only up to apertures of about f/11.0.
Use of such conventional lens systems requires a larger than desired illumination system to provide excessive light at the object platen for insuring the projection of a sufficient quantity of light to the exposure station. The disproportionately large, expensive and heavy illumination system is required due to the low light gathering power of a lens system having an f/11.0 aperture. The size, cost and weight of the illumination system results not only from the lamps but also from the electrical power supply required to energize the lamps at closely spaced intervals to obtain high speed copying. Further, it is difficult to distribute the large quantity of light energy evenly across the object platen.
It has been determined that a lens system with an aperture having a much lower f-number of such as 5.6 would significantly transmit more light energy to the work station by about a factor of four, easing the requirements of the illumination system and improving overall system performance.
Such a moderate aperture lens system must maintain its cost proportionate to the remainder of the copying machine and be well corrected for aberrations to maintain copy quality.
These requirements might point toward production using the techniques of injection molding optical plastic or resinous materials. The optical plastic materials available, however, are considerably lower in refractive index than the glass materials used in conventional lens systems demanding that the lens elements be specially designed to correct for aberrations
Further, the lens system must be easy to assemble and install so that unskilled labor may be used to reduce costs.
Additionally, the lens system should be non-vignetted. A vignetted lens system will transmit decreasing quantities of light energy from the center of the object and moving towards the edges of the object. Thus a vignetted lens system will transmit less light energy from the edges of an object than from the center of that object. In photocopier applications, this vignetting can be corrected by providing increased illumination energy at the fringes of the object platen. This can be difficult to achieve, however, especially where high intensity illumination is required at the center of the object platen.
Heretofore it has not been deemed commercially feasible to achieve a lens system which obviates the disadvantages stated above. The invention herein succeeds in achieving this end.